A television picture is produced by deflecting one or more electron beams across a display screen in a particular pattern. The electron beams strike light-producing phosphor elements to produce a raster. The electron beam intensity is modulated in response to video signal information to produce the desired picture.
Deflection or scanning of the electron beam or beams is provided by a deflection yoke comprising vertical and horizontal deflection coils. Deflection current flowing through the coils generates an electromagnetic field around the coils in the vicinity of the electron beams. The deflection currents are controlled to produce appropriate deflection fields in order to properly deflect the electron beams to generate the desired scanned raster.
During the horizontal trace interval, the main operating components of the horizontal deflection circuit are the horizontal deflection coils of the deflection yoke and the "S-shaping" capacitor, which corrects symmetrical scan nonlinearity caused by the picture tube geometry. The amount of deflection energy necessary to scan a given horizontal line is placed into the yoke at the beginning of each trace interval. This energy circulates in a resonant manner during the trace interval from the yoke into the "S-shaping" capacitor and back into the yoke. The losses in the circuit, if not compensated, cause the circulating energy to decrease during the trace interval, resulting in a smaller deflection current amplitude at the end of trace than at the beginning of trace. This appears as a compressed picture at one side of the screen. This asymmetrical horizontal linearity error is caused by the sum of the losses in the horizontal deflection circuit. Most of these losses occur as resistance losses in the deflection yoke.
To correct this asymmetrical linearity error, several circuit solutions are known. One solution is to use a saturable inductor in series with the yoke. This inductor is in saturation during the second half of the trace interval resulting in an increase in the rate of change of the deflection current; i.e., beam scan velocity. The amount of linearity correction is adjusted by varying the saturation point of the inductor. This adjustment is difficult and in many cases, good linearity is not obtainable because of the large tolerance of the saturable inductor. The use of a saturable inductor may introduce a deflection current offset. A raster centering circuit may then be required. A further disadvantage of this arrangement is the expensive construction of the saturable inductor.
Another solution to linearity correction uses a series-connected variable voltage source which adds some power to the deflection circuit to correct the linearity error. Since the deflection current also passes through the voltage source, a solution of this type is not energy efficient.